The oxygen binding curve (commonly called the "oxygen dissociation curve") for hemoglobin is formed by the measurements of the fraction of total hemoglobin that is oxygenated as a function of the partial pressure of oxygen (PO.sub.2) to which the hemoglobin sample is exposed. The entire curve and/or parameters derived from it are of substantial physiological and clinical significance. Currently used techniques in this field employ dual wavelength photometry, and such techniques comprise passing time-shared measure and reference beams .lambda..sub.M and .lambda..sub.M through the sample while it undergoes oxygenation and utilizing the differences in absorption of these beams as measured by a photomultiplier tube and associated circuitry for deriving the desired "oxygen dissociation curve", which is in fact an oxygen association curve.